Now you can apply the Mean Value Theorem to the middle expression, which states that, if [itex]f\left( x \right)[/itex] is differentiable over (a,b) and continuous over [a,b], there exists a [itex]c \in \left( {a,b} \right)[/itex] so that:
[tex]f'\left( c \right) = \frac{{f\left( b \right) - f\left( a \right)}}{{b - a}}[/tex]